Some types of electronic equipment exercise operation control of an input/output device by changing the value of a register of the input/output device with a CPU (Central Processing Unit). To change the value of a register, the CPU normally needs an operation of three steps. The three steps are reading the value of the register, changing the read value, and writing the changed value to the register.
Further, a method of completing an operation to change the value of a register with the CPU in one step is known. According to the method, some bits of write data, out of a plurality of bits, are used to cause an input/output device to select one write mode from a plurality of write modes. The input/output device selects a write mode based on some bits of write data received from the CPU. The input/output device changes the value of a register based on the selected write mode and the remaining bits. Since processing performed by the CPU is accomplished in one step according to this method, the time needed to change the value of a register is reduced when compared with a method by which the CPU performs the processing in three steps.
The following is a document that discloses issues concerning the register control method:
Japanese Laid-open Patent Application No. 2004-46483
However, the method by which part of write data is used to specify the write mode has a problem in that all of the bits of write data cannot be used to change the value of a register, leading to decreased efficiency. This is because some bits of write data of the plurality of bits are used to cause the input/output device to select the write mode and only the remaining bits are used as bits to change the value of a register.